1. Field of the Invention
This invention relates to magnetic data storage disks and, in particular, to a technique for improving the tracking of a magnetic head while it is writing or reading data on the disk.
2. Description of the Related Art
Magnetic disks are used by computers for storing data. Conventional magnetic disks are made by sputtering a magnetic layer of a material such as a Pt/Co alloy onto an aluminum or glass disk substrate. Before the disk may be used, it is necessary to initialize the magnetic layer to record track addresses and synchronization marks. A magnetic head is then used to write and read data on the disk in circular tracks.
The initialization process is usually performed on each disk individually using a magnetic servo writing machine. As the track density of the disk (i.e., the number of tracks per unit of radial distance) is increased, this process becomes more and more time consuming. Moreover, highly accurate mechanical and environmental tolerances must be attained to produce disks with high track densities. Both of these factors in turn tend to increase the cost of the disks.
An alternative to the extremely accurate but expensive initialization process now used for high track density disks would overcome these problems and help to reduce the cost of the disks.
The prior art attempted to solve this problem by several techniques which use markers of various types to locate the position of the track on the disk. In the method described in U.S. Pat. No. 4,961,123, for example, a plurality of concentric grooves are formed on the surface of the disk. A light beam is focused on the disk. The focused beam is optically diffracted by the grooves, and a push-pull tracking signal is obtained from the diffracted beam pattern, as described in U.S. Pat. No. 4,856,108. This arrangement is less than satisfactory because the inclusion of an optical head significantly increases the size and cost of the apparatus.
The methods described in U.S. Pat. No. 4,746,580 and U.S. Pat. No. 4,935,278 involve removing a portion of the magnetic recording layer by ion milling or reactive ion etching to form markings which can be read by a magnetic head. This process stains the disk surface and may cause head crashes during the reading or writing of data on the disk.